1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus capable of preventing excessive temperature rise of a heating roller.
2. Description of Related Art
A fixing apparatus using a heating method is widely used to fix toner, which is used for an image forming apparatus such as a copying machine or a printer, onto recording paper. A fixing apparatus using a heating method is provided with a heating roller, which has heating means such as a heater, and a press roller, which is in contact with the heating roller with pressure, and is constructed to convey and support recording paper, which has a toner image transferred thereto, between the heating roller and the press roller, and to melt and, further, pressurize toner on the recording paper, so as to fix the toner image onto the recording paper.
It is necessary for such a fixing apparatus to reliably melt toner on the recording paper and to precisely control the surface temperature of the heating roller in order to avoid bad influence on the recording paper. Conventionally, the surface of the heating roller is pressed by a plurality of thermistors and the temperature of a plurality of positions on the surface of the heating roller is detected to control electric conduction to the heater, so as to maintain an even surface temperature of the entire heating roller.
It is however necessary to press a thermistor with a suitable pressure when a thermistor is used for measuring the surface temperature of the heating roller precisely. Consequently, such a fixing apparatus has a problem that the surface of the heating roller deteriorates due to friction between the thermistor and the surface of the heating roller and the fixing strength deteriorates when the thermistor is pressed against one position of the heating roller. Such a fixing apparatus also has another problem that the temperature cannot be detected precisely when dirt on the surface of the heating roller adheres to the surface of the thermistor.
In order to solve these problems, a fixing apparatus and an image forming apparatus provided with a non-contact temperature sensor have been suggested (see Japanese Patent Application Laid-Open No. 2000-227732 and Japanese Patent Application Laid-Open No. 2001-109316, for example). Known as a non-contact temperature sensor is one having an infrared ray sensor, which detects the surface temperature of the heating roller by detecting radiant heat (infrared ray) from the surface of the heating roller with the infrared ray sensor.
A non-contact temperature sensor described in the above patent documents includes a thermistor for infrared ray detection and a thermistor for temperature compensation. The thermistor for infrared ray detection detects an infrared ray to be radiated from the surface of the heating roller and the output voltage thereof depends on the ambient temperature (i.e. the temperature of the thermistor for infrared ray detection itself). It is necessary to detect the temperature of the thermistor for infrared ray detection itself in order to compensate such temperature dependence. The thermistor for temperature compensation is therefore placed in a position, which is near a sensor for infrared ray detection, free of the influence of the infrared ray to be radiated from the surface of the heating roller. The temperature of the surface of the heating roller can be obtained indirectly by fetching output voltage values of the two thermistors arranged as described above and carrying out an operation with operation means such as a CPU.
However, these thermistors, which are different from a contact temperature sensor in that the temperature of the surface of the heating roller is not observed directly, have a problem that excessive temperature rise of the heating roller cannot be detected immediately.